1. Field of the Invention
The present invention relates to a testing apparatus, a testing method, a jitter filtering circuit and a jitter filtering method. More specifically, the invention relates to a testing apparatus for testing a device-under-test such as a semiconductor circuit.
2. Related Art
Conventionally, there has been a case of testing whether or not an electronic device such as a semiconductor circuit can correctly operate with an input signal having a predetermined eye opening. For example, a minimum eye opening called as an eye mask is specified as a characteristic which an electronic device having a serial I/O interface should have in order to guarantee quality of a signal outputted out of a transmitter or a receiver's tolerance to an input signal influenced by disturbance. Eye mask stipulated by PCI Express may be exemplified as the eye opening which the output signal of the transmitter should meet and the eye opening of the input signal that should guarantee that the receiver will normally operate.
Generally, the eye opening must be measured by taking characteristics of a clock recovery circuit of the receiver into account. It is because the receiver having the serial I/O interface for example recovers clock from a data signal transmitted from a transmitter and re-samples the data signal by using the recover clock.
FIG. 15 is a diagram showing a configuration of a conventional receiver 400. The receiver 400 receives a data signal transmitted from a device-under-test 200 on the transmitter side. A clock recovery circuit 410 generates recovered clock based on the data signal. A data sampler 420 re-samples the data signal corresponding to the recovered clock to generate recovered data.
FIG. 16 is a graph showing one exemplary filtering characteristic of the clock recovery circuit 410. When a signal-under-measurement is inputted to the clock recovery circuit 410, generated clock will have jitter transfer characteristic corresponding to the filtering characteristic. Then, when the signal-under-measurement is re-sampled by using the recovered clock, the recovered data result in containing jitter components having frequency characteristic obtained by subtracting frequency characteristic of the recovered clock from frequency characteristic of the jitter components contained in the signal-under-measurement.
Because the receiver 400 is thus influenced by the recovered clock whether or not it can correctly re-sample the data signal, the eye opening of the data signal must be measured by taking the characteristics of the clock recovery circuit 410 into account.
For example, PCI Express stipulates for the measurement of the eye opening of a data signal by using clock recovery function (see Equation (1)) which is a 1st-order high-pass filtering characteristics with 1.5 MHz cut-off frequency.
                                                        H              3                        ⁡                          (              s              )                                =                      s                          s              +                              ω                3                                                    ,                              ω            3                    =                      2            ⁢                                                  ⁢            π            ×            1.5            ×                          10              6                                                          Eq        .                                  ⁢                  (          1          )                    
Conventionally, testing of eye opening of a serial I/O interface such as PCI Express have been carried out by means of a real-time sampling oscilloscope or a testing apparatus having a clock recovery function. For example, the real-time oscilloscope samples a data signal-under-test at high speed and measures jitter in the data signal-under-test by using digital signal processing. Then, it applies the clock recovery function described above to the measured data to analyze an eye diagram.
The testing apparatus recovers clock from a data signal-under-test by using the clock recovery circuit and tests an eye diagram by comparing logic values of the data signal-under-measurement with the edges of the expected values at the clock used as timing reference.
However, the real-time oscilloscope has had a problem that it requires to accurately measure edge timings of the data signal-under-test and to sample the data signal-under-test at high sampling rate. Accordingly, it has had a problem that its apparatus cost is high. Still more, because it is difficult to be adapted to multi-channel, it is unable to test a multi-lane device having several tens to several hundreds of signal paths disposed in parallel. Furthermore, it is hard to use it in a device high volume-production test because it takes time in analyzing the eye diagram by using the data signal processing.
The testing apparatus using the clock recovery circuit also has to meet the requirements: a) assuring the accuracy of timing of the edges of the recovered clock and b) tracking the timing fluctuation by accurately generating recovered clock, so that its circuit complexity increases. Therefore, its apparatus cost has been high and it results in inflexibility of the function of the clock recovery circuit.
Accordingly, it has been desired to realize an apparatus capable of testing and evaluating devices having a high-speed and multi-lane serial I/O interface at low cost.